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1. LECTURE # 7
Genetics part 2

2. GENE LINKAGE
 Every organism possesses numerous
characters controlled by thousands of genes,
but the number of chromosomes is limited.
Therefore, each chromosome must carry
many genes on it.
 All the genes located on the same
chromosome are linked to each other. This
phenomenon of staying together of all the
genes of a chromosome is called linkage.
Gene linkage is a physical relationship
between genes.

3. LINKAGE GROUP
 A chromosome carries its linked genes en
bloc in the form of a linkage group.
 The number of linkage groups corresponds to
the number of homologous pairs of
chromosomes. Man has 23 linkage groups.
 If genes are linked on sex chromosome, their
linkage is called sex linkage.
 If genes are linked on autosomes their
linkage is called autosomal linkage.

4. INHERITANCE OF LINKED GENES
 Genes for color blindness, hemophilia, gout
etc. form one linkage group on human X -
chromosome.
 Similarly, gene for sickle cell anemia, leukemia
and albinism make another linkage group on
human chromosome 11.
 Linked genes whose loci are close to each other
do not obey Mendel’s law of independent
assortment, because these cannot assort
independently during meiosis. Gene linkage
also minimizes the chances of genetic
recombination and variations among offspring.

5. DETECTION OF LINKED GENES
 Gene linkage can easily be detected by
performing test cross between two gene pairs.
 In such type of cross, a heterozygous individual
for two traits (F1) is back crossed with its
recessive parent (P1).
 If all phenotypic combinations (parental or
recombinants) are produced in equal 1:1:1:1
ratio then there would be no linkage between
genes.
 If this ratio is deviated i-e., if only parental
types are produced, complete linkage is
believed.

6.  In typical dihybrid cross, the complete or
tight linkage inhibts the outcome of
recombinant types and diturb 9:3:3:1 ratio of
independent assortment , as a result only
parental combinations are produced in 3:1.

7. CROSSING OVER
 Linked genes can be separated by crossing
over.
 Closer the two gene loci, more strongly are
their genes linked.
 The farther apart two genes lie, greater are
chances of their separation through crossing
over.
 Crossing over is an exchange of segments
between non-sister chromatids of
homologous chromosomes during meiosis.

8. CHIASMATA
 Chiasmata are formed at many places
between non-sister chromatids of
 homologous chromosomes. Crossing over
occurs at 4 strand stage between non-sister
 chromatids. It may take place at more than
one place along a chromosome.
 Exchange of chromosome segments logically
means exchange of DNA, i.e. genes or alleles.
 As alleles of non-sister chromatids are
different, an exchange between their
segments results in recombination of genes.

9.  Chiasmata are formed at many places
between non-sister chromatids of
homologous chromosomes.
 Crossing over occurs at 4 strand stage
between non-sister chromatids. It may take
place at more than one place along a
chromosome.

12. CROSS OVER OR
RECOMBINATION FREQUENCY
 It is the proportion of recombinant types
between two gene pairs as compared to the
sum of all combinations.
 The recombination frequencies between two
linked genes can be calculated by
backcrossing the heterozygote to a
homozygous double recessive.

14. RECOMBINATION FREQUENCY
 The recombination frequency is directly proportional
to the distance between the linked gene loci. Genes
can be mapped on a chromosome on the basis of their
recombination frequencies.
 If 1% of recombination frequency is equal to 1 unit map
distance, the two linked genes A and B with a 20%
recombination frequency must be 20 units apart.
 Importance of crossing over:
 Crossing over produces genetic variations among
offspring. Genetic variations lead to tremendous
variations in their traits.
 Variations provide raw material for evolution by letting
them adapt successfully to the changing environment.

16. SEX LINKAGE IN DROSOPHLLA
• Modern understanding of genetic linkage
came from the work of Thomas Morgan.
• Morgan showed that two recessive genes in
Drosophila melanogaster, white eye (w) and
miniature wing (m) are linked.

17. FRUIT FLY:
 The fruit fly, Drosophila melanogaster has eight chromosomes
in the form of four homologous pairs.
 T.H. Morgan (1911) noticed a peculiar difference in the
chromosomes
 of male and female Drosophila. The chromosomes of the three
homologous pairs were similar in both of the sexes, but the
fourth pair was very different.
 The female had two similar rod shaped X-chromosomes in the
fourth pair, while the male had one rod shaped X-chromosome
but the other a morphologically different, J-shaped
 Y chromosome in the fourth heteromorphic pair. X and Y
chromosomes are called sex chromosomes
 because these have genes for determination of sex.
Chromosomes of the other three pairs are autosomes. All
chromosomes other than sex-chromosomes are called
autosomes. Autosomes do not carry any sex determining gene.

18.  In his initial test cross aimed at exploring the
precise relationship between eye color and
sex, Morgan bred white-eyed males (Xw
Y)
with wild-type red-eyed females (X+
X+
). This
cross yielded only red-eyed offspring

19. Next, Morgan decided to cross two flies from the
F1 generation—specifically, a red-eyed female (X+
Xw
)
and a red-eyed male (X+
Y)—to test for a recessive
pattern of inheritance.
As shown in the table, the offspring of this cross
exhibited a 3:1 ratio of red eyes to white eyes, which
indicated that white eyes were recessive. Moreover,
all of the white-eyed F2 offspring were male.

20. Next, as previously discussed, Morgan conducted a third
cross to determine whether white eyes were lethal in female
flies. Here, he bred red-eyed females (X+
Xw
) with white-eyed
males (Xw
Y).
This third cross revealed that white eyes were in fact
not lethal in females, because it produced a 1:1:1:1
ratio of red-eyed females to white-eyed females to red-
eyed males to white-eyed males.

21.  Because this cross yielded all white-eyed males and
all red-eyed females, Morgan could indeed conclude
that the white-eye trait followed a sex-linked pattern
of inheritance.
Morgan crossed a white-eyed female with a red-eyed male.

22. NOBEL PRIZE
 Morgan’s discovery of sex linked inheritance
was a great contribution in the understanding
of genes and chromosomes. In 1933,T.H.
Morgan was awarded a Nobel prize for his
contribution to genetics.

23. SEX LINKAGE IN HUMANS
 Humans have 46 chromosomes in the form of 23 pairs.
 22 pairs are of autosomes and one pair is of sex-
chromosomes. Autosome pairs are common in both the
sexes but the 23rd sex chromosome, pair is very
different in males and females.
 A woman has two similar X chromosomes in her 23rd
pair but a man has an X chromosome along with a
much shorter Y chromosome in his 23rd pair. The 23rd
pair in man is heteromorphic.
 She is XX but he is XY.
 SRY is the male determining gene. It is located at the
tip of short arm of Y-chromosome.
 Its name SRY stands for “Sex determining regions of Y.”

24.  There are about 200 genes present on human
X chromosome.

25. X LINKED TRAIT
 A trait whose gene is present on X
chromosome is called X linked trait.
 X linked traits are commonly referred as sex
linked traits.
 A gene present only on X chromosome,
having no counterpart on Y chromosome, is
called X linked gene.

26. X LINKED DOMINANT
TRAIT
It is a trait which is determined by X linked
dominant gene.
It is more common in females than males. All
daughters of an affected, but none of his son
are affected.
e.g: hypophosphatemic rickets, Incontinentia
pigmenti etc

27. HYPOPHOSPHATEMIC
RICKETS
 It is a rare X linked dominant trait.
 It doesn’t result from vitamin D deficiency but
its cause is a genetic communication failure at
molecular level. The gene encoding bone
protein never receive vitamin D‘s message to
function.

29. X LINKED RECESSIVE
TRAIT
 X linked recessive is a trait that is
determined by X linked recessive gene.
 Pattern of inheritance is from grandfather to
daughter and then grandson.
e.g:
 Haemophilia A and Haemophilia B
 Colour blindness
 Testicular feminization

30. HAEMOPHILIA
 X-linked recessive
 3 types
 Transfer in zigzag pattern
 More common in male
 Haemophilia a:
 absence of clotting factor 8 (x linked
recessive)
 Most common type
 80% in population

31.  Haemophilia b:
 absence of factor9 (x linked recessive)
 Second major type
 20% in population
 Haemophilia c:
 absence of factor 11 (autosomal)
 Negligible in population

32. HISTORY OF HAEMOPHILIA
 The Haemophilia is called royal disease
because haemophilia gene was passed from
Queen Victoria, who became Queen of
England in 1837 to ruling families of Russia,
Spain and Germany. Queen Victoria’s gene of
haemophilia was caused by spontaneous
mutation.

33. PEDIGREE ANALYSIS
 A diagram of family history that uses
standardized symbols.
 A record of inheritance of certain traits for
two or more generations presented in the
form of a diagram or family tree is called
pedigree.

37. TESTICULAR
FEMINIZATION SYNDROME
 Testicular feminization syndrome is a rare X
linked recessive trait. Although the person
affected by this trait have a male set of XY
chromosomes, yet tfm gene on their X
chromosome develops them physically into
females. They have breast, female genitalia, a
bling vagina but no uterus. Degenerated testis
are also present in abdomen. Such individuals
are happily married as females but are sterile.
It is an androgen insensitivity syndrome. Male
sex hormone testosterone has no effect on
them.

38. Y LINKED TRAIT
 A trait whose gene is present on Y chromosome is called Y linked trait.
 e.g: SRY gene, hypertrichosis (growth of hair on ear pinna), porcupine
man (straight hair on body), webbing of toes etc.
 SRY gene on Y chromosome of man determines maleness. It is a male
sex switch which triggers developmental process towards maleness
after 6 week pregnancy.
 Y chromosome is not completely inert. It does carry a few genes which
have no counterpart on X chromosome. Such genes are called Y linked
genes.
 e.g: testis determining factor (TDF), Minor histocompatibility gene. (H-Y)
 Y linked traits are found only in males.
 These traits directly pass through Y chromosome from father to son only.
 The Y linked inheritance is also called holandric inheritance.
 The concepts of dominant and recessive don't apply to Y linked traits, as
only one allele is ever present in any (male) individual.

40. PSEUDOAUTOSOMAL GENE
Some genes are present on X and Y
chromosomes both they are called X and Y
linked genes. They are also called
pseudoautosomal genes.
e.g:
 Bobbed genes in drosophila.
Their pattern of inheritance is like autosomal
genes.

41. SEX LIMITED TRAIT
 A sex limited trait is limited to only one sex due
to anatomical differences. Such trait affect a
structure or function of the body present in only
males or only in females. These traits may be
controlled by sex linked or autosomal genes.
 e.g: gene for milk yield in dairy cattle affect only
cow, beard growth in humans is limited to men.

42. SEX INFLUENCED TRAIT
 Sex influenced trait occurs in both males and
females but it is more common in one sex. It is
controlled by an allele that is expressed as
dominant in one sex but recessive in the other.
This difference is due to hormonal difference
between the sexes.
e.g: pattern baldness, amount of hair on body,
muscle mass etc.
 pattern baldness is a sex influenced trait . Many
more men than women are bald. It is inherited
as an autosomal dominant trait in males but as
an autosomal recessive trait in females.